Image tubes are electro-optical devices that are used to detect, intensify, and shutter optical images in the near ultraviolet, visible, near infrared, X-ray and gamma-ray regions of the electro-magnetic spectrum. They are used extensively for night vision, astronomy, X-ray and gamma-ray intensification, electron microscopy, medical research, radiology, and as high speed light shutters. Basically, an image intensifier tube comprises a photocathode for the conversion of the incident radiant image to a low energy electron image, an electron lens for the production of a high energy electron image, and a phosphor screen for the conversion of the high energy electron image to a light image. The photon gain of the image intensifier tubes may range from less than unity to several million, depending on the incident radiation wave length and the number of the tube stages.
In many applications, remote monitoring of the output of the photocathode is desirable. Although the visual image from the phosphor screen may be transmitted to a remote location using fiber optics, the quality and resolution of the image decays as a result of its propagation through fiber optic elements. Hence, a digital output of the image transmitted from the photocathode is preferable, since it may be transmitted along an electrical conductor without denigration.
Several types of digital photoelectronic devices are capable of digitizing the output of the photocathode, including charge coupled devices (CCDs), charge injected devices (CIDs) and MOS photodiode arrays. However, previous attempts to mate the digital devices with the photocathode have been inadequate for many applications.
In one previously proposed device, the image tube is fabricated with a short fiber optic lead connected to the phosphor screen. A CCD or other digital device may be optically bonded to the other end of the fiber optic lead to produce a digital output. However, this system suffers from limited resolution, linear and shear distortion due to the fiber optics, and light losses due to optic coupling and phosphor efficiency. Furthermore, this system does not overcome any of the disadvantages associated with phosphor screens, such as problems associated with the decay time of phosphor and color matching the color of the phosphor screen to the spectral sensitivity of the digital device.
Another attempt at mating a digital device with the image tube has comprised replacing the phosphor screen with the digital device itself. However, since vacuum photo tubes undergo a twelve-hour bake-out process at 375.degree. C., the digital devices are damaged as a result of the fabrication process of the image tube The most successful attempts at producing a system of this type known to applicant have resulted in image tubes with lifetimes on the order of a only few days.
Thus, a need has arisen for a image tube with direct coupling between the photocathode and the digital device, which overcomes the high temperature limitations of the fabrication process.